Small Craft Advisory 193, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 PM AKST Sat Mar 3 2018

.ANALYSIS AND UPPER LEVELS...
The progressive upper level flow continues today with the trough
moving across south central Alaska and another warm upper level
ridge over the eastern Bering Sea moving into the southwest
mainland of Alaska. The surface low well south of Kodiak Island
has continued to play a larger part in the weather today as it has
set the stage to advect additional moisture toward Kodiak and
Cook Inlet. This additional moisture, combined with a region of
isentropic lift over the Kenai Peninsula has increased the amount
of snow over the Kenai Peninsula and Kodiak Island. Farther west,
the next low center is near the Kamchatka Peninsula which is
pushing the 250 mb jet stream over the Aleutian Islands and
southern part of the Bering Sea. This will all help keep the
progressive pattern over the area for the next few days.

&&

.MODEL DISCUSSION...
There is not a whole lot to gripe about with the models today with
synoptic features in good alignment through the middle of next
week. One item of note is that the trough moving through
south central Alaska today is slower than what any of the models
were showing yesterday. Forecasts have been adjusted to reflect
this. Models are still in agreement that there will be a good
push of colder air moving into the region by next weekend which
will break us out of this progressive pattern we have been in.

&&

.AVIATION...
PANC...Snow will taper off to showers late afternoon to early
evening time frame and conditions should improve back to VFR
(though ceilings may remain under 5000 ft). With the snow moving
out and a warm ridge building in, it sets the stage for the
development of fog or stratus overnight. At this time, the
stratus looks more likely with the inversion height and if it is
stratus, it may last through most of the day Sunday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Sun and
Mon)...
One snow event will wrap up tonight before we set the stage for
another one Mon. In between, we will see a brief offshore flow
event. First though, the upper level trough is evident on
satellite clearing through the Susitna Valley at this time. Snow
is diminishing with this feature from west to east. With some
(albeit weak) cold air advection working in with it, we could see
snow continue to fall into the early evening for East Anchorage,
the Hillside, and out towards the Matanuska Valley. The upper
level energy will then shift into the Copper River Basin where a
couple of inches will fall into the early morning hours. The
shortwave will leave plenty of low-level moisture in its wake.
Thinking remains much the same that with weak cold air advection
across the Cook Inlet region and some dry northerly flow in the
boundary layer, stratus will be more likely than fog tonight.
However, there always seems to be fog potential when near the open
waters of the Knik Arm. Fog looks a bit more likely for the
Copper River Basin at this time as the trough will exit that area
in the middle of the night and there is little to clear out the
lingering moisture.

As colder air filters in from the north and west Sun, it will
bring another round of outflow winds. This will favor places like
Whittier and Seward, but Valdez and Thompson Pass will also see
gusty conditions develop. These winds will not be anywhere near as
strong as earlier this week due to a lack of any significant upper
level support.

By Monday, another shortwave moves across the area from west to
east. Yesterday's model runs showed this as a positively tilted
trough with scattered forcing. Today's runs were slightly more
impressive with the feature and some better energy digging into
the backside of the trough. For this reason, have opted to leave
"likely" snow in the forecast for a brief period for now. This
system will struggle for moisture, so we don't expect more than
about an inch in any population center at this time.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 2)...
(Tonight through Monday night)
High pressure will continue building across the southwest mainland
tonight leading to clearing skies across most of the area. The
next system approaching the region can be seen on satellite moving
across the central Bering Sea and Pribilofs, which should reach
the west coast by late Sunday morning/early afternoon. As the
front comes ashore, gusty winds and snowfall could lead to periods
of blowing snow along the Kuskokwim Delta Coast Sunday afternoon.
The front will move across the region Sunday evening through
Monday morning bringing snow to most areas, with total
accumulations between 1-3" favoring the Kuskokwim Delta this time.
High pressure builds in again over the region again on Monday
bringing clearing skies and cooler morning temperatures, which
will feel colder over the Kuskokwim Delta due to a stiff west wind
most of the day.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 2)...
(Tonight through Monday night)
A progressive storm pattern will continue across the Bering Sea with
a front pushing across the central Bering Sea/Aleutians this
evening bringing a quick shot of precipitation and gusty winds to
the Pribilofs and central and eastern Aleutians. Warm air
advecting from the south out ahead of the front will lead to
mostly rain though snow may mix in Sunday as colder air filters in
behind the front. A transitory ridge builds in Sunday night ahead
of a storm force low moving into the western Bering Sea/Aleutians.
This low is expected to be a bit stronger than the previous lows
this week hence the stronger winds (50 kt) expected Sunday
night/Monday morning. Strong westerly gale force winds will also
bleed in behind the low due to strong cold air advection on the
backside of the system. These westerly winds will race across the
Bering Sea Monday night and will have a long fetch, thus waves
will rise to 20-30 ft over the western half of the Bering Sea.

&&

.LONG TERM FORECAST (Days 3 through 7)...

A fast moving frontal system will cross the eastern Bering Sea
Monday evening, to push onshore over southwest Alaska late Monday
night through Tuesday morning. Snow and blowing snow will likely
be a concern for the Kuskokwim Delta and western Capes with snow
spreading further inland mid morning through early afternoon.
While the upper level trough will be weakening as it crosses into
south central Alaska, snow looks likely there as well with light
snow developing Tuesday afternoon then increasing and becoming
more widespread Tuesday afternoon and Wednesday. Following this
system, the strong westerly jet stretching across the Bering Sea will
continue to bring a very active storm track through the end of
the week. A large upper level low will dig southeast across
mainland Alaska around Friday spreading additional cold air across
the state and strong, cold north to northwesterly flow across the
central and eastern Bering Sea, the Alaska Peninsula and eastern
Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 178. Gale Warning 411.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...JR